The invention relates to a process for the thermal conversion of hydrogen sulfide, contained in a gaseous stream, with sulfur dioxide to elemental sulfur wherein the gaseous stream containing hydrogen sulfide is introduced into the reactor together with sulfur dioxide obtained from a waste gas purification facility arranged downstream of the reactor. The thus-produced vapor-phase sulfur is condensed out by cooling and obtained as a product, and the gaseous stream extensively freed of sulfur compounds is discharged and fed into a downstream waste gas purification facility.
One process step frequently necessary in the processing of raw gas streams is the separation of sour gases, essentially CO.sub.2, H.sub.2 S and mercaptans. This separation can be conducted by various techniques, for example by adsorption or scrubbing. In this connection, it is of special advantage to remove the carbon dioxide and the sulfur-containing sour gases separately from the raw gas streams. Suitable for this purpose are chemical as well as physical scrubbing processes, the latter, in particular, being preferred, especially where the raw gas streams have a high CO.sub.2 content. The residual fraction obtained, for example, in an H.sub.2 S-selective scrubbing operation and enriched with H.sub.2 S contains usually between 25 and 90 mol-% of H.sub.2 S, depending on the hydrogen sulfide content of the gaseous stream to be cleaned.
It has been known for a long time that sulfur can be obtained from a gaseous stream thus enriched with H.sub.2 S. This can take place, for example, in accordance with a sulfur-producing facility (Claus plant) based on the Claus reaction EQU 2 H.sub.2 S+SO.sub.2 .fwdarw.3/x S.sub.x +H.sub.2 O+.DELTA.H
(wherein x=1, 2, 3, 4, 5, 6, 7 or 8).
Heretofore, the use of a catalyst has been indispensable in sulfur production plants on an industrial scale. The advantages of the catalyst reside in its activity at lower temperatures permitting high sulfur yields by virtue of improved equilibrium conditions at low temperatures. With corresponding initial outlay, the yields of sulfur are limited to about 99.5 mol-%. A substantial drawback of these methods, however, lies in the sensitivity of the catalysts employed--especially in processes designed for maximum yield. More specifically, certain chemical compounds which occur with relative frequency in the feed gases, such as, for example, ammonia compounds, can lead to clogging and an accompanying rapid deactivation of the catalyst. Also any traces of oxygen bring about sulfating and catalyst deactivation.
DOS 3,403,651 discloses a special process for the catalytic conversion of hydrogen sulfide contained in a gaseous stream to elemental sulfur by using sulfur dioxide. For this purpose, the preheated gaseous stream is conducted for conversion purposes over a catalyst bed wherein a temperature is maintained of 125.degree.-450.degree. C. The thus-formed vapor-phase sulfur is condensed out by cooling, and the gaseous stream, extensively freed of sulfur compounds, is exhausted to the atmosphere. For maximizing the sulfur yield, the gaseous stream is passed over the bed in a way entailing only minimum pressure losses. Furthermore, the catalyst bed is cooled internally by a cooling medium. Based on this mode of operation, the sulfur yield can be increased to close to the theoretically maximum possible value.
Several other variations of the Claus process are known wherein the conversion of the gaseous stream containing hydrogen sulfide to elemental sulfur takes place by catalytic methods. These catalytic conversion processes required a very closely maintained stoichiometry as a pre-condition for achieving high yields. However, under practical circumstances, the quantity, as well as the composition of the feed gas, fluctuate so that the the optimum operating point, is frequently missed and the theoretically possible yield cannot be attained.
Catalyst beds charged with sulfur are also susceptible to fire; for this reason, the feed gas stream must be kept free of oxygen. Since the feeding and discharge operations cannot be performed with sour gas, natural gas is used instead. The latter is burned close to stoichiometry in order to be able to provide a maximally oxygen-poor and hot inert gas for the sulfur production plant. Only in this way is it possible to prevent oxygen from passing into the reactors wherein the accumulated sulfur otherwise would burn, in case of oxygen introduction, spontaneously with a very hot flame which would lead to extensive damage.